With the development of radio and space technologies, several satellites based navigation systems have already been built and more will be in use in the near future. One example of such satellites based navigation systems is Global Positioning System (GPS), which is built and operated by the United States Department of Defense. The system uses twenty-four or more satellites orbiting the earth at an altitude of about 11,000 miles with a period of about twelve hours. These satellites are placed in six different orbits such that at any time a minimum of six satellites are visible at any location on the surface of the earth except in the polar region. Each satellite transmits a time and position signal referenced to an atomic clock. A typical GPS receiver locks onto this signal and extracts the data contained in it. Using signals from a sufficient number of satellites, a GPS receiver can calculate its position, velocity, altitude, and time.
A GPS receiver has to acquire and lock onto at least four satellite signals in order to derive the position and time. Usually, a GPS receiver has many parallel channels with each channel receiving signals from one visible GPS satellite. The acquisition of the satellite signals involves a two-dimensional search of carrier frequency and the pseudo-random number (PRN) code phase. Each satellite transmits signals using a unique 1023-chip long PRN code, which repeats every millisecond. The receiver locally generates a replica carrier to wipe off residue carrier frequency and a replica PRN code sequence to correlate with the digitized received satellite signal sequence. During the acquisition stage, the code phase search step is a half-chip for most navigational satellite signal receivers. Thus the full search range of code phase includes 2046 candidate code phases spaced by a half-chip interval. The carrier frequency search range depends upon the Doppler frequency due to relative motion between the satellite and the receiver. Additional frequency variation may result from local oscillator instability.
Coherent integration and noncoherent integration are two commonly used integration methods to acquire GPS signals. Coherent integration provides better signal gain at the cost of larger computational load, for equal integration times.
The signals from the navigational satellites are modulated with navigational data at 50 bits/second. This data consists of ephemeris, almanac, time information, clock and other correction coefficients. This data stream is formatted as sub-frames, frames and super-frames. A sub-frame consists of 300 bits of data and is transmitted for 6 seconds. In this sub-frame a group of 30 bits forms a word with the last six bits being the parity check bits. As a result, a sub-frame consists of 10 words. A frame of data consists of five sub-frames transmitted over 30 seconds. A super-frame consists of 25 frames sequentially transmitted over 12.5 minutes.
The first word of a sub-frame is always the same and is known as TLM word and first eight bits of this TLM word are preamble bits used for frame synchronization. A Barker sequence is used as the preamble because of its excellent correlation properties. The other bits of this first word contains telemetry bits and is not used in the position computation. The second word of any frame is the HOW (Hand Over Word) word and consists of TOW (Time Of Week), sub-frame ID, synchronization flag and parity with the last two bits of parity always being ‘0’s. These two ‘0’s help in identifying the correct polarity of the navigation data bits. The words 3 to 10 of the first sub-frame contains clock correction coefficients and satellite quality indicators. The 3 to 10 words of the sub-frames 2 and 3 contain ephemeris. These ephemeris are used to precisely determine the position of the GPS satellites. These ephemeris are uploaded every two hours and are valid for four hours to six hours. The 3 to 10 words of the sub-frame 4 contain ionosphere and UTC time corrections and almanac of satellites 25 to 32. These almanacs are similar to the ephemeris but give a less accurate position of the satellites and are valid for six days. The 3 to 10 words of the sub-frame 5 contain only the almanacs of different satellites in different frames.
The super frame contains twenty five consecutive frames. While the contents of the sub-frames 1, 2 and 3 repeat in every frame of a superframe except the TOW and occasional change of ephemeris every two hours. Thus the ephemeris of a particular signal from a satellite contains only the ephemeris of that satellite repeating in every sub-frame. However, almanacs of different satellites are broadcast in-turn in different frames of the navigation data signal of a given satellite. Thus the 25 frames transmit the almanac of all the 24 satellites in the sub-frame 5. Any additional spare satellite almanac is included in the sub-frame 4.
The almanacs and ephemeris are used in the computation of the position of the satellites at a given time. The almanacs are valid for a longer period of six days but provide a less accurate satellite position and Doppler compared to ephemeris. Therefore almanacs are not used when fast position fix is required. On the other hand, the accuracy of the computed receiver position depends upon the accuracy of the satellite positions which in-turn depends upon the age of the ephemeris. The use of current ephemeris results in better position estimation than one based on non-current or obsolete ephemeris. Therefore it is necessary to use current ephemeris to get a precise satellite position and hence the receiver position.
A GPS receiver may acquire the signals and estimate the position depending upon the already available information. In the ‘hot start’ mode the receiver has current ephemeris and the position and time are known. In another mode known as ‘warm start’ the receiver has non-current ephemeris but the initial position and time are known as accurately as in the case of previous ‘hot start’. In the third mode, known as ‘cold start’, the receiver has no knowledge of position, time or ephemeris. As expected the ‘hot start’ mode results in low Time-To-First-Fix (TTFF) while the ‘warm start’ mode which has non-current ephemeris may use that ephemeris or the almanac resulting in longer TTFF due to the less accurate Doppler estimation or time required to download new ephemeris. The ‘cold start’ takes still more time for the first position fix as there is no data available to aid signal acquisition and position fix.
In a normal GPS receiver operation, the GPS timing is determined after acquiring a satellite signal and demodulating the navigation data in the satellite signal. The HOW word in the navigation data gives the time of the week. The time indicated by the HOW word of a frame corresponds to the start of the next sub-frame. Since the sub-frame duration is six seconds, the HOW increases by 1 for every six seconds or sub-frame. This time represents the time at which the particular sub-frame is transmitted from the satellite. Since very accurate atomic clocks are used in the satellites, the time represented by the navigation data is also accurate. Further, clock correction coefficients are also transmitted as a part of the navigation data and this is used to further improve the accuracy of the satellite derived timing. Therefore, this timing is usually used to determine the time duration for the satellite signal to travel from the satellite to the receiver and hence the pseudorange.
However, in some cases the satellite signal may be very weak and it may not be possible to demodulate the navigation data. A possible scenario is when the signal is acquired and bit edges are synchronized, but the bit polarity, i.e., whether the bit is 0 or 1, can not be determined. Under this condition the HOW word and the next sub-frame start point can not be determined. As a result the time determination is not possible. In addition to this, a highly accurate Real Time Clock (RTC), which may have a stability of 20 PPM or more, may not be available to the receiver. Under these conditions the receiver clock may be synchronized with a base station clock in an aided GPS system as explained in the U.S. Pat. Nos. 6,417,801, and 6,734,821, and published US application 2003/0107513. The total pseudorange is also based on the sub-millisecond pseudorange, but these references do not disclose the advantage of using Doppler measurements. These references include measurements from five satellites to solve the three co-ordinates of the receiver position, the receiver clock offset and the reference satellite integer number one millisecond ambiguity. On the other hand another U.S. Pat. No. 7,064,709 takes the Doppler into amount, but, like the previous set of references, does not take into account the bit synchronization. U.S. Pat. No. 6,215,442 and U.S. Pat. No. 5,812,087 disclose using the relative velocity between the SPS receiver and satellite to determine the system time and pseudorange residuals. In another embodiment, two overlapping data records are used to determine the time. U.S. Pat. No. 6,052,081 discloses using two overlapping data records to determine the time. U.S. Pat. No. 6,191,731 discloses determining the location based on linearized pseudoranges using a velocity enhanced method. U.S. Pat. No. 6,670,916 makes use of grid points and is applicable in cold start conditions. The system disclosed in this patent may also use third party data. U.S. Pat. No. 6,191,731 discloses a system based on velocity vectors. Published US application US 2005/0225483 uses assisted data and change of sampling rate.
Therefore, there is a need for system and methods that enable a navigation receiver to determine receiver position without the need for timing information from navigational satellites or aiding systems, thereby allowing the navigation receiver to operate under weak received signal conditions.